In recent years energetic research has been conducted on so-called energy harvesting techniques for recovering electric power from waste energy, such as vibrations and heat, which normally is merely dissipated. For example, utilization is anticipated in vibration-generated power, which can be used as a power source for automobile tire pressure gauges and ubiquitous sensor networks, for example. On the other hand, thermoelectric generated power is applied only in niche fields such as for example watches which utilize the temperature differences between the human body and the environment. One reason for this is the low performance of the thermoelectric conversion materials used in thermoelectric elements. The performance index (or figure of merit) of thermoelectric conversion materials is expressed byZ=S2/(ρ·κ)  equation (1)
Here S is the Seebeck coefficient (μV/K), ρ is the resistivity (Ω-cm), and κ is the thermal conductivity (W/(m·K)). The units of the figure of merit Z are K−1, and as the efficiency of a thermoelectric element using a thermoelectric conversion material, often ZT, the product of the figure of merit and the usage temperature T (K), is used. The efficiency ZT of a conventional thermoelectric element using bulk material as the thermoelectric conversion material is approximately 1. It is thought that if a thermoelectric conversion material with efficiency ZT exceeding 3 could be fabricated, such a material could be substituted for compressors and other cooling systems used for example in refrigerators.
Satisfactory thermoelectric conversion materials have a large Seebeck coefficient S and low values of both resistivity and thermal conductivity. Bi2Te3 is a semimetal material which is representative of thermoelectric conversion materials currently in use. However, Bi2Te3 system materials are toxic and have a high environmental impact. Hence oxide materials which can be used as safer thermoelectric conversion materials have been attracting interest of late. One such material is SrTiO3, a petrovskite type oxide with an ABO3 structure, doped with La at the A sites (Non-Patent Reference 1: T. Okuda et al, Phys. Rev. B, Vol. 63, 113104 (2001)). This material is a degenerate semiconductor with the n-type conduction.
Further, there have been theoretical proposals of methods to reduce the dimensions of the nano-scale structures of thermoelectric conversion materials, in order to improve the efficiency ZT of thermoelectric elements. Thermoelectric conversion materials have actually been trial-fabricated having a two-dimensionalized structure by using superlattices, and having a one-dimensional structure using whiskers. In these thermoelectric conversion materials, by reducing the dimensionality of the material microstructure and manipulating the state density distribution of conduction carriers, both an increase in the Seebeck coefficient S and reduction of the resistivity ρ are achieved. Further, by reducing the dimensionality of the microstructure of a thermoelectric conversion material, phonon scattering is increased and the thermal conductivity κ is lowered. That is, in a thermoelectric conversion material having a microstructure of reduced dimensionality, an increase in the value of the above-described figure of merit Z is attained. In particular, it has been reported that by adopting Bi2Te3 as a thermoelectric conversion material and fabricating a two-dimensionalized structure, the efficiency ZT has exceeded 2.
Further, it has been reported that even when adopting an oxide as a thermoelectric conversion material, by utilizing a two-dimensionalized structure, the efficiency ZT is increased (see Non-Patent Reference 2: H. Ohta et al, Nature Mater., Vol. 6, 129). Non-Patent Reference 2 reports that a superlattice comprising an n-type thermoelectric conversion material in which the B sites of SrTiO3 were doped with Nb and insulating SrTiO3 had an efficiency ZT of approximately 0.3, that when limited to the superlattice interfaces in particular the efficiency ZT exceeded 2, and that Ca3Co4O9 (a p-type thermoelectric conversion material) whiskers exhibited an efficiency ZT of approximately 1. Through advances in thin film technologies in recent years, even in the case of oxide thermal conversion materials, fabricating superlattices (two-dimensional structures) such as those described above using oxides is not difficult. However, if an oxide is adopted in an attempt to fabricate a high-efficiency thermal conversion element, a number of problems arise. First, if an oxide is adopted, it is difficult to fabricate a one-dimensional structure, having dimensionality lower than two dimensions. Specifically, in order to fabricate a one-dimensional structure in an oxide, one conceivable method is to adopt a quantum wire which uses as a template a step of a step-terrace structure of a substrate. However, in order to use this method, fabrication conditions must be controlled such that film deposition in what is called step flow mode is realized. These fabrication conditions are conditions to always cause thin film growth only from the step portion, and consequently the process window is narrow and precise condition control is necessary. As a separate problem, linearity of substrate steps is not always ensured, and there is the problem that the linearity of quantum wires fabricated using such steps as a template is similarly not ensured.
As a method to resolve these problems, Patent Reference 1 (Japanese Patent Application Laid-open No. 2004-296629) discloses a method in which a single-crystal substrate inclined by a slight angle (from 0.2 to 15°) from an arbitrary face orientation is used. In this method, step bunching perpendicular to the inclination direction which is formed on the single crystal is utilized (see for example paragraph [0013] in Patent Reference 1). However, judging from the contents of the disclosure, what is formed on the inclined single-crystal surface disclosed in Patent Reference 1 are ordinary steps, and not bunched steps. In Patent Reference 1 it is stated that a buffer layer surface of a nonconductive material, fabricated on the single crystal inclined by a slight angle (0.2 to 15°), is used as the specific means of forming bunched steps, and that the bunching of steps formed can be used to fabricate thermoelectric conversion material with a wire structure. However, in the disclosure of Patent Reference 1, the reason for formation of bunched steps on the single crystal inclined by a slight angle (0.2 to 15°) is not disclosed.
Patent Reference 1: Japanese Patent Application Laid-open No. 2004-296629
Non-Patent Reference 1: T. Okuda et al, “Large thermoelectric response of metallic perovskites: Sr1-xLaxTiO3 (0<x<0.1)”, Phys. Rev. B, Vol. 63, 113104 (2001)
Non-Patent Reference 2: H. Ohta et al, “Giant thermoelectric Seebeck coefficient of a two-dimensional electron gas in SrTiO3”, Nature Mater., Vol. 6, 129 (2007)
The method of Patent Reference 1 above has the following problems. That is, (1) because step bunches are not formed on the substrate, a buffer layer must be formed. Further, (2) the linearity in the direction of extension in the bunched steps formed on the buffer layer is not ensured as the linearity in normal steps is not ensured. Hence even if bunched steps were formed, using the method of Patent Reference 1, a wire shape would not necessarily be determined with stability. Further, (3) in order to form in a wire shape the thermoelectric conversion member comprising thermoelectric conversion material extending along bunched steps, fabrication conditions must be controlled precisely so as to result in a step flow mode. Specifically, if there is two-dimensional growth in the thermoelectric conversion material not only from the step edge but also from the terrace surface, there are such problems as irregularity in the intervals between the thermoelectric conversion member wires, and merging of steps, so that wire widths change. In order to prevent such problems and cause growth to occur in step flow mode, the conditions of fabrication of the thermoelectric conversion material must be controlled with high precision.
This invention was devised in light of the above-described goals. By providing a thermoelectric conversion structure which can be fabricated easily and with good reproducibility to take the electronic structure of a quasi-one-dimensional structure, that is, a wire structure, this invention contributes to improvement of the performance of thermoelectric elements adopting such thermoelectric conversion structures.